Induction heating is well known in the art and has conventionally been achieved by means of apparatus which constitutes a primary winding of a transformer with the bearing ring forming the secondary winding. This is accomplished by providing a horseshoe construction for the primary winding and having a connecting piece to complete the circuit, the connecting piece being adapted to receive the bearing in inductive contact.
Many types of induction heaters are presently in use. Their use, however, is limited by several disadvantages derived from the fact that induction heating presently involves passing a high-power current, often of several kilowatts, through an inducting coil to effect high heat in the conducting metal; and the primary winding is generally of normal supply frequency and is generally of substantial size which makes it difficult to transport. Another disadvantage associated with prior art arrangements is that the bearing requires to be demagnetized during or after the heating operation.
In an example of the prior art, U.S. Pat. No. 2,836,694 (Emerson) discloses an induction heating method by saturating a magnetic workpiece with maximum magnetic flux density to effect the heating process. The DC power source is used so as not to overload the RF generator at temperatures below the Curie point of the workpiece. The present invention seeks to utilize low flux density and is concerned with conductive workpieces. Whereas Emerson relies on magnetization of the workpiece, the present invention seeks to minimize or eliminate the magnetization of the workpiece.
In another example of the prior art U.S. Pat. No. 3,187,155 (Beckert) recognizes the necessity for a separate demagnetization step but he achieves this with a bulky and expensive arrangement of a motor and variable voltage transformer to remove the magnetism created by the application of a low frequency (60 Hz) current in the initial heating step. By a careful choice of parameters the present invention achieves the heating step without having to resort to an additional demagnetizing step which is not only expensive and time consuming but also involves the use of bulky non-portable equipment.
U.S. Pat. No. 4,311,896 (Junya) uses a 60 Hz current to excite a coil 2 surrounding core 4 by a connection 10. Junya also fails to recognize the advantages of the present invention which uses certain parameters and items such as a switched mode power supply at high frequency in a controlled manner.
Japanese patent 53-43646 uses eddy currents to heat objects in order to weld them together; whereas the present invention uses the workpiece as a secondary to produce circulatory currents in the workpiece. The Toyota invention is not applicable to the heating of bearings--in fact, if it was used to heat bearings, the heating effect would only be obtained in the zone including the gap with the result that a bearing would merely be welded together in such a zone. It is an object of the present invention to heat a bearing so that in can be fitted to a shaft--it does not seek to weld a few bearing balls to the bearing bed.
It is an object of the present invention to obviate some of the disadvantages of the prior art and to provide apparatus which is easily portable and which does not magnetize a bearing or ring unduly, thereby avoiding the necessity for providing a demagnetization step and apparatus therefor. It will be appreciated that a bearing must remain free of magnetism to prevent attraction of metallic particles which could cause considerable damage.